This invention relates to apparatus for monitoring the concentration of toner in a developer mix for use in developing equipment of an electrostatic-printing system, an electrophotographic system etc.
In an electrostatic-printing system wherein carrier particles and toner are mixed in a fixed proportion and are used as a developer mix, it is desirable that the mixing ratio (or toner concentration) in the developer mix between the carrier particles and the toner be controlled to be substantially constant. To this end, the toner concentration in the developer mix needs to be detected accurately.
For example, in apparatus for monitoring and controlling the concentration of toner in a developer mix as disclosed in U.S. Pat. No. 3,572,551, a sampling tube and a return tube are disposed at the bottom of a reservoir. A screw is mounted within the return tube, and the developer mix accumulated in the bottom of the reservoir slides through the sampling tube and up is passed through the sampling return tube by the rotation of the screw. A sampling coil is wound on the outer periphery of the sampling tube. The toner concentration of the developer mix inside the sampling tube is detected in the form of the inductance of the sampling coil.
Such apparatus can precisely measure the toner concentration under fixed conditions. However, it is prone provide to erroneous measurements due to fluctuations in conditions, e.g., the temperature and the humidity. This is because, on account of the changes of the temperature and the humidity, the quantity of slide of the developer mix varies and the quantity of movement or the density of the developer mix inside the sampling tube varies.
Also Japanese Patent Laying-open No. 52-125339 discloses apparatus for detecting the concentration of toner, in which a developer mix is introduced into the hole of a hollow bobbin and toner concentration is detected by a detection coil wound on the hollow bobbin. As in the foregoing apparatus, however, such apparatus has the disadvantage of liability to erroneously measure the toner concentration of the developer mix on account of fluctuations in the temperature, the humidity etc.
In addition, another apparatus for detecting the concentration of toner is known from Japanese Patent Laying-open No. 52-125338.